1. Field of the Invention
The present invention relates to input buffers and, more particularly, to input buffers with biasing to minimize the effect of common mode variation of signals and maximize output gain.
2. State of the Art
Input buffers are typically optimized for speed around small input voltage ranges which are susceptible to contamination by noise. Accordingly, removal of noise from the input signals is desirable for improved isolation of the desired signal. Input buffers that include differential inputs may be used to transmit signals over relatively large distances with minimal signal degradation due to interference from external sources (i.e., noise or other interference). The resulting differential output signals from a conventional input buffer exhibit both an original logic signal and a complementary logic signal of the original input signal. Common Mode Variation (CMV) manifests as equal changes of both signals in a differential transmission. CMVs may be caused by many things including noise, power supply or reference voltage differences. Therefore, differential input buffers should offer the best possible rejection of CMV in order to recreate a signal that more closely approximates the original input signal.
One common problem associated with input buffers that generate differential output signals is ensuring a desirably balanced output signal. FIG. 1A illustrates a schematic of a conventional input buffer 100 depicted as a fully differential operational amplifier. The input buffer 100 receives an input voltage Vin+ at a positive terminal and a complementary input voltage Vin− at a negative terminal. The input buffer 100 outputs differential outputs Vout+ and Vout− based on the input voltages. When noise couples to the input signals as presented to the input buffer, the levels of the respective input signals may individually and uniquely be affected thereby creating variations in phase and magnitude on the differential output signals. For example, FIG. 1B illustrates the differential outputs Vout+ and Vout− as generated from a conventional input buffer 100. As shown, the outputs Vout+ and Vout− are not balanced. Specifically, the output signal Vout− should be the substantial inverse of Vout+ such that the crossing points 150 of the two output signals Vout+ and Vout− in combination would result in a signal of substantially 0 volts. Many factors, including process variations, may also cause Vout+ and Vout− to not be balanced even without noise.
FIG. 2 illustrates a circuit diagram of a conventional input buffer 200 that is driven with differential input signals Vin+ and Vin− and generates differential output signals Vout+, Vout−Input buffer 200 includes a first stage circuit 280 and a second stage circuit 290. In a conventional input buffer, the circuit is self-biased to reduce the affects of CMV on the input signals. A self-biased input buffer may have relatively small CMV sensitivity when implemented in a traditional long channel process. However, for MOS devices in modern processes, the threshold voltages represent a larger portion of the circuit operating voltage and the CMV sensitivity of such an implementation may become significant.
As illustrated, the first stage circuit 280 includes a first input transistor 201 for receiving the input signal Vin+ at a gate terminal 201′, a second input transistor 202 for receiving the input signal Vin− at a gate terminal 202′, first and second bias transistors 203, 204 having associated gates 203′, 204′, and third and fourth load transistors 207, 208 with associated gates 207′, 208′.
The gates 203′, 204′ of the first and second bias transistors 203, 204 are electrically connected to each other and the gates 207′, 208′ of the third and fourth load transistors 207, 208 are electrically connected to each other. Also, a drain terminal of each of the third and fourth load transistors 207, 208 is respectively electrically connected to a drain terminal of each of the first and second input transistors 201, 202. The first stage circuit 280 further includes an ouput node 209 at which positive output Vout+ is generated. The output node 209 is electrically connected to the drain terminal of the fourth load transistor 208 and to the drain terminal of the second input transistor 202. In addition, first stage circuit 280 includes connecting the drain of third load transistor 207, gates 207′, 208′ of third and fourth load transistors 207, 208 with the gates 203′, 204′ of first and second bias transistors 203, 204 at the node 220 to form the self-biasing characteristic of the input buffer 200.
The second stage circuit 290 includes a third input transistor 250 for receiving the output voltage Vout+ at a gate terminal 250′, fifth and sixth bias transistors 252, 253 having associated gates 252′, 253′ and seventh and eighth load transistors 254, 255 with associated gates 254′, 255′. The gates 252′, 253′ of the fifth and sixth bias transistors 252, 253 are electrically connected to each other and the gates 254′, 255′ of the seventh and eighth load transistors 254, 255 are electrically connected to each other. Also, a drain terminal of the seventh load transistor 254 is electrically connected to a drain terminal of the third input transistor 250, and separately a drain terminal of the eighth load transistor 255 is electrically connected to a drain terminal of the sixth bias transistor 253.
The second stage circuit 290 further includes an output node 256 at which output Vout− is generated. In addition, second stage circuit 290 is also self-biased by connecting the drain of seventh load transistor 254, the gates of seventh and eight load transistors 254, 255 with the gates 252′, 253′ of fifth and sixth bias transistors 252, 253 at the node 260.
The configuration of the conventional input buffer generates unbalanced positive and negative output signals, such as that illustrated in FIG. 1B. Furthermore, manufacturing process variations result in variations to the transistor switching points thereby injecting further variations in the alignment and propagation of the output signals. Accordingly, there is a desire for an input buffer that generates a better balanced differential output.